Erosion Control November/December 2016 : Page 34

With no need for geogrid reinforcement, the wall required less excavation. stormwater pipes came into the pond, and one outlet was located at the low end. The retention pond was surrounded by a failing retaining wall. The wall had been built 21 years before, and was made of dry-cast segmental retaining wall blocks. The wall was leaning forward, and some blocks had deteriorated. It was easy to tell that further destruction was ahead, possibly leading to damage to the parking lot and obstruction of the stormwater facility. Replacement of the wall was needed, but there were major challenges to be overcome. The project would have to have a minimal impact on the office complex and the parking lot. No geogrid installation would be possible because it would require more excavation than space allowed. Using geogrids would have required about 78 inches of excavation at the higher points, meaning removal and then replacement of a large portion of parking lot, substantially increasing the disruption for the office workers and dramatically increas-ing the cost. The retaining wall would have to be designed to handle constant water immersion and the possibility of heavy water load in front of the wall. It would also have to be durable against parking lot deicing chemicals that would run off into the retention pond. The engineer for the project was Curt Derichs, principal with Civil Design Professionals in Bloomington, MN. “Water is the biggest enemy of retaining walls,” he says. Now he would have to design a wall that would have water against 34 EROSION CONTROL the face. The design would also have to ensure that water did not get trapped behind the wall, causing an early failure. The choice for this replacement retaining wall was a ReCon gravity wall, which could be installed quickly without geogrid reinforcement. The blocks have the durability needed to last in a retention pond with deicing products. At the start of the project, the retention pond was dewa-tered. The pond had to be drained several times during construction. The failing wall was removed, and a leveling pad of crushed stone was put in place and compacted. The base block was placed 45 inches deep. Clean, one-inch stone was backfilled into the spaces between the units to provide free drainage. ReCon Series 50 units were chosen for this wall. They are manufactured from wet-cast, air-entrained concrete, making them the best choice for an environment of road salts, water immersion, and freezing and thawing. They come in a variety of sizes, including 48 inches wide, 16 inches high, and depths from 24 to 60 inches. They weigh from 1,411 to 3,115 pounds. Forterra Pipe and Precast of Shakopee, MN, supplied the blocks. For this design, 24-inch, 39-inch, and 45-inch deep blocks were used. Successive block courses were added, with geo-textile fabric placed between rows, and the voids were filled with clean stone. The blocks are placed on the lower level and then pulled forward so that the groove slides onto the lower WWW.EROSIONCONTROL.COM RECON